[matthewmacleod]

"The fish's pond"? "The fishes' pond"?

That's a pond belonging to a single fish ("The fish's pond") versus one belonging to multiple fish ("The fishes' pond"). You're completely right!

[coldpie]

It's hard because the plural form of "fish" is usually "fish." I think we allow "fishes" when "fish" is ambiguous, like my example. Maybe we should just change to "fishes" all the time :)

[alex_young]

Deep in the weeds here, but the plural of a single species of fish is 'fish', while multiple species of fish are 'fishes'.

'Swimming with the fishes' means you must have wound up on the wrong side of a boat in a diverse body of water. It would be a shame to have to 'swim with the fish'. So ambiguous.

[coldpie]

Learn something new every day :)

[js2]

The plural "fish" is more than one fish of the same species. The plural "fishes" is multiple species. At least biologists make that distinction. edit: what Alex Young said.

[csn]

"Fishes" is the word used to describe different species of fish.

Edit: late for the party. "Deeres" and "sheepes" follow the same pattern, I presume.